1. Field of the Invention
This invention relates to a disc skew detecting apparatus for detecting the skew of a disc in which information signals are written and read out with an optical pickup device.
2. Description of the Related Art
In the technical field of recording and reproducing data in and from a recording medium such as an optical disc, a small-sized optical disc having a large capacity, which enables high density for recording and reproducing data, such as, high-definition static and dynamic images, is demanded. To satisfy the above performance conditions, a technique for shortening the wavelength of a laser beam radiated from an optical pickup device and increasing the numerical aperture (NA) of an objective lens so as to reduce the spot size of the laser beam has been broadly researched.
However, in an optical disc device having a smaller spot size of the laser beam, the track pitch of the optical disc is narrowed. As a result, when the objective lens of an optical pickup device has an optical axis skewed in relation to the optical disc, greater interference of information signals from adjacent tracks, that is greater crosstalk and interference of preceding and succeeding in information signals than in conventional disc recording and reproducing system, is caused. Consequently, it is more difficult to record and reproduce information signals precisely when the optical disc device has a narrower track pitch as demanded by the high density recording.
The relationship of the tolerance to skew of an optical disc device of wavelength .lambda. of the laser beam and the numerical aperture NA is expressed in the following equation. EQU Skew Tolerance=.lambda./(NA).sup.3 ( 1)
For this reason, a precise skew servo must be realized in attaining higher density of the optical disc, and therefore a skew detecting apparatus capable of precisely detecting the actual skew amount in recording and reproduction is required.
A conventional skew detecting element is showing in FIG. 1 as including a reflecting skew sensor (called a tilt sensor) comprised of a light emitting diode 21 and a photodetector 22. The reflecting skew sensor detects the disc skew in one direction only via the position of the reflected light from the disc.
As is clearly shown in FIG. 2, the reflecting skew sensor shown in FIG. 1 is mourned slightly away from an objective lens 24 and an actuator 25 of the optical pickup device, with a measuring direction m of the skew sensor parallel to the radial direction r of the disc. Thus, this reflecting skew sensor does not take account of the curvature of the actual track.
For this reason, the spot of the laser beam (hereinafter referred to simply as the beam spot) for recording and reproducing information signals radiated from the objective lens of the optical pickup device is deviated from the skew measuring position of the reflecting skew sensor. The deviation includes a deviation due to the difference between the track on which the laser beam spot is located and the track on which the skew is detected, a phase deviation of rotation synchronous components of the skew caused by the disc rotation, and a deviation in the skew measuring direction due to the difference in the track curvature on the disc. These deviations generate errors in the skew amount detected by the reflecting skew sensor having its skew measuring direction parallel to the radial direction. Thus, with the conventional reflecting skew sensor, it is difficult to precisely detect the skew amount of the disc on the beam spot radiated by the optical pickup device.